A dash, a leap, a mid-air twist, fists swinging down from over the head into a trademark power-pose landing: The sight of Cristiano Ronaldo celebrating his first goal in a Juventus shirt was at once familiar and foreign, old moves transposed to a new context.

For supporters of the club, it was above all reassuring. There had been no sense of a panic for a team that won its opening three games without Ronaldo scoring, yet a quiet sense of insecurity did linger. What if he really was struggling to adjust at 33 years old? What if their team could not serve him as effectively as Real Madrid had?

Such questions have been replaced by bullish confidence, just in time for Juventus to launch its Champions League campaign away to Valencia. It is the competition that Ronaldo describes as “my home,” the one that he was signed to win. In his first official interview after joining, with the broadcaster Dazn, he spoke of an ambition to do so within the next three...